This invention relates to superconducting electromagnet apparatus, especially those for deflecting charged particle beams such as electrons, which are provided with a magnetic shield for confining the leakage fields.
FIG. 18 is a plan view of a conventional superconducting deflection electromagnet apparatus, which is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 2-174099. FIG. 19 is a sectional view of the superconducting deflection electromagnet apparatus along the line A--A in FIG. 18, as viewed in the direction of the arrows. FIG. 20 is a perspective view of the superconducting deflection electromagnet apparatus of FIG. 18. Within a magnetic shield 11 is disposed a cryostat 4, in which is accommodated the deflection coil assembly: main coils 1, quadrupole correction coils 31, and sextupole correction coils 32. When excited, the coils 1, 31, and 32 generate a magnetic field as represented by the magnetic field line 12. The beam duct (not shown), inserted into the cavity 70, extends between the upper and the lower coil assemblies.
The method of operation of the superconducting deflection electromagnet apparatus is as follows. The superconducting coils 1, 31, and 32 are excited to produce a magnetic field represented by the magnetic field line 12. The charged particle beam proceeding through the beam duct between the upper and the lower coil assemblies is deflected 180 degrees by the Z-component (see the coordinate axes shown in the figures) of the magnetic field. The magnetic field line 12 extending out of the cryostat 4 is confined substantially within the magnetic shield 11. The magnetic shield 11 thus shields in the magnetic field leaking out of the cryostat 4. Since the magnetic field line 12 extends through the magnetic shield 11, an electromagnetic force acts between the coils and the magnetic shield 11.
The above conventional superconducting deflection electromagnet apparatus, however, has the following disadvantage. The reservoir (not shown) for the liquid helium is disposed within the cryostat 4. Thus, when a large quantity of the liquid helium is to be held in the reservoir, the size of the cryostat 4 and hence that of magnetic shield 11 surrounding it become greater. Further the magnetic shield 11 is a heavy construct of a large volume, so that its construction and assembly is difficult.
Furthermore, in the case of the above superconducting deflection electromagnet apparatus, errors in the relative position between the coils and the magnetic shield 11 may result from the production inaccuracy, the thermal shrinkage of the parts produced at the normal temperature and then cooled to a very low temperature, and the deformation of the support structure due to the electromagnetic force acting among the coils. Since the relative position between the coils and the magnetic shield 11 is not adjustable, the errors in the relative positions cause deviations from the design values in the electromagnetic forces acting between the coils and the magnetic shield 11. Usually, the relative position between the coils and the magnetic shield 11 is designed to minimize the electromagnetic force acting therebetween. Thus, when the relative position is deviated from the design value due to an error, the electromagnetic force acting between the coils and the magnetic shield 11 may become too large for the support structure.